1. Field of the Invention
The present invention relates to the field of fusing materials using a laser, and particularly to soldering metallic objects such as electrical connections between a printed circuit board and an electrical component.
2. Background Information
U.S. Pat. No. 5,580,471 to Fukumoto et al. and U.S. Pat. No. 4,963,714 to Adamski et al. exemplify several conventional automated laser soldering techniques.
The Fukumoto patent discloses an automated laser soldering device that includes a semiconductor diode which is used to generate a plurality of laser beams, which are transmitted and combined using optical fibers. The exit ends of the optical fibers are directed toward a mirror which reflects the laser light emitted from the optical fibers into a lens assembly. The lens assembly focuses the laser light onto objects to be soldered together, so that the laser light can heat the objects and melt solder to fuse the objects together. The mirror or scanner is rotated to change an angle of incidence between the laser light and the mirror's surface, and thereby reflect the laser light to different locations on an object to be soldered such as a printed circuit board. Instead of simply moving the laser beam across the object when moving the beam from one soldering location to another soldering location as in other conventional laser soldering methods, the laser is turned off while the mirror rotates, so that locations other than soldering locations will not be struck and heated by the beam. An off-axis camera and image processing equipment are also provided to help monitor and control operation of the laser soldering device.
The Adamski patent discloses an automated laser soldering system that includes a plurality of laser diodes, each with an optic fiber for directing the output of the diode to a soldering location. The laser diodes are organized into a plurality of groups. For each group, the outputs of the optic fibers for the laser diodes in the group are combined to provide a single beam. The single beam outputs of the groups are simultaneously focused on different soldering locations on a printed circuit board, to simultaneously form solder Joints at the different soldering locations.
In conventional automated laser soldering techniques where the laser beam is focused on a soldering location so that the beam only strikes a component to be soldered and not, for example, a portion of a circuit board the component is located on, it can be difficult to control an intensity of the beam to prevent overheating of the components to be soldered while also ensuring that the temperatures rise high enough for the solder to properly flow into the joint and bond with the components. Furthermore, the beam must be accurately located on the component to be soldered, because when the beam spot is only just large enough to cover the component to be soldered, slight misalignments of the beam can cause the beam to partially or completely miss the component to be soldered. However, if the beam spot is made larger than the component area to be soldered to reduce sensitivity to misalignment, then the excess portion of the laser beam will strike and can damage other objects such as a non-metallic printed circuit board supporting the component.
In the electrical device industries, hand soldering techniques are often used instead of automated soldering techniques in specific situations for a variety of reasons. These reasons include, for example, physical or mechanical constraints, high temperature sensitivity of components to be soldered, and avoidance of tooling costs. However, product quality can vary unacceptably when hand soldering techniques are used due to such factors as operator error, and natural operator variations from solder joint to solder joint. For example, even a highly skilled human operator will apply a soldering iron in a slightly different way for a slightly different time duration at each solder joint. In situations where the operator also manually applies a quantity of solder, the quantity of solder applied to each joint can also vary. Furthermore, hand soldering techniques are typically slow in comparison to automated soldering techniques. These drawbacks magnify as a distance or pitch between adjacent soldering locations decreases.
Accordingly, a need exists for an automated soldering technique that can be used to reliably and quickly solder temperature-sensitive components without causing damage to the components, while simultaneously providing better test yields than conventional hand soldering techniques.